1. Field of the Invention
The present invention relates to scintillation counting and more particularly to novel high efficiency scintillation counting compositions useful in "dry" counting elements.
2. State of the Prior Art
Scintillation counters are used for measuring the density or concentration of emissions from radioactive sources, such as beta particles, gamma radiation, etc. Such counters are well known in the art and the principles on which they operate are described, for example, in "Source Book on Atomic Energy" by Samuel Glasstone at pages 140-142, and "Two Liquid Scintillation Neutron Detectors," by Muelhouse and Thomas, Nucleonics 11, 44 (1953). Briefly, these counters detect and quantify emissions from scintillator compositions which comprise a solvent (liquid or solid) which "captures" the incoming radiation to be detected and measured, a primary fluor which responds to the incoming, "captured" radiation by fluorescing at a specified wavelength, and if desired, a secondary fluor or wave shifter which responds to the emissions of the primary fluor by fluorescing at a specified second wavelength.
There are three types of scintillator compositions. These are: (1) solid scintillators comprising a crystal of a solid hydrocarbon material; (2) liquid scintillators which comprise one or more suitable solid scintillators dissolved in a liquid solvent; and (3) so-called solid solution scintillators which comprise a solid scintillator in a solid polymeric solution. The compositions of this invention are most closely related to solid solution scintillators.
The most commonly used commercial solid solution scintillators comprise, e.g., a polystyrene block having fluors included therein. In use, samples are analyzed, for example, for .beta.-particle emission, by dissolving the sample in a suitable solvent, for example, toluene; applying the solvent solution of sample to the solid solution scintillator block; and scintillation counting, i.e., quantifying the fluorescent emissions from the block. Unfortunately, such scintillators can only be used with very strong radiation emitters such as .sup.60 Co, .sup.137 Cs and UV excitation, with certain very specific organic solvent sample systems of weak .beta.-emitters such as .sup.14 C, tritium or the like, or pure .gamma.-emitters such as .sup.125 I or the like. The utility limitations of these systems are apparently due in large part to an inability to achieve the intimate contact between the emitter and the fluor or scintillator. Such intimate contact is required if these short range radiations are to affect the fluor. Thus, the efficiency of such counting systems is reduced. As a point of reference, the counting efficiencies of such prior art solid solution scintillators generally range below about 20% of theoretical maximum, apparently due to the lack of emitter-fluor intimacy. Liquid scintillator compositions on the other hand are capable of efficiencies above 35% and in some cases 100% of theoretical, most probably due to the intimate emitter-fluor contact possible in a liquid medium. Use of liquid scintillator compositions, however, involves a large number of handling problems well known to those skilled in the art.
The following patents further illustrate the above-noted developments in scintillation counting compositions.
U.S. Pat. No. 3,010,908, issued Nov. 28, 1961, discloses the use of dialkyl styrene polymers as the primary absorber in a solid solution scintillation counting composition. The maximum weight percent of fluors which can be carried as a solute in such a solvent system is disclosed to be about 5%.
U.S. Pat. Nos. 2,985,593 and 3,356,616, disclose styrene-derived monomers polymerized or copolymerized with vinyl or methacrylate monomers to form the solvent for a solid solution scintillation counting composition. The fluors are carried as solutes, and the fluor concentrations do not exceed about 3 weight percent.
U.S. Pat. No. 3,457,180, issued July 22, 1969, discloses as the solvent for a solid solution scintillator, copolymerized p-vinyltoluene and methyl methacrylate. The disclosed amount of fluor dissolved in such a solvent is less than 3 weight percent.
U.S. Pat. No. 3,150,101, describes the formation of scintillating ion exchange beads by the suspension-polymerization of polyvinyltoluene or polystyrene crosslinked with divinylbenzene, the monomers containing the fluors dissolved in them. Such beads, by virtue of their large size, and therefore their small surface area per unit of weight, do not have high counting efficiencies for low-energy radiation.
U.S. Pat. No. 3,513,102 discloses a fluorescent coating in which a fluor and a copolymer of an acrylate and styrene is dissolved in an organic solvent, and the solution is emulsified in an aqueous dispersion of a hydrophilic colloid. The copolymer is not derived from a latex, but is a solution polymer isolated, redissolved and blended by high-speed milling for dispersion in a gel binder.
U.S. Pat. No. 3,418,127, issued to A. G. Millikan Dec. 24, 1968, describes a technique for increasing the efficiency of direct electron recording compositions by incorporating increased levels of fluor into solid polymer solutions. Specifically the fluor is dispersed in an aqueous dispersion of monomer and then the monomer is emulsion polymerized. According to the teachings of this reference, maximum fluor concentrations on the order of 12-15% (col. 4, line 15) are achievable. Such compositions are formed into coatings as part of a composite photographic element suitable for the detection and recording, for example, of x-rays and other high energy emissions. No suggestion is made to use the coated element as a scintillation counter for, in particular, low energy emissions such as .beta.-particles.
Other documents of interest are as follows:
Commonly owned U.S. patent application Ser. No. 506,919 filed by T. J. Chen on Sept. 17, 1974, now abandoned, entitled "Uniform, Efficient Distribution of Hydrophobic Materials Through Hydrophilic Colloid Layers, and Products Useful Therefor," refiled as Ser. No. 575,689 on May 8, 1975, describes a novel technique for incorporating unusually large concentrations of hydrophobic materials, for example, color forming couplers, into polymeric particles derived from a latex. Gelatin photographic elements are formed, for example, by a process which involves the step of
(a) forming an aqueous dispersion by intermixing the hydrophobic material and an aqueous polymeric latex, optionally including gelatin in the dispersion; PA1 (b) forming a wet layer by coating onto a suitable support the aqueous dispersion from step (a); and PA1 (c) thereafter removing a substantial proportion of the water from the wet layer through which the hydrophobic material is dispersed. U.S. Ser. No. 575,689 has been refiled and divided into the following U.S. Applications: Ser. Nos. 744,680 filed Nov. 24, 1976; 778,182 filed Mar. 16, 1977 and 778,184 filed Mar. 16, 1977. PA1 (a) provide polymeric particles which are compatible with the water-miscible solvent (i.e., they do not coagulate or precipitate when the latex is gradually blended into the solution of hydrophobe in the water-miscible organic solvent); PA1 (b) preferably are compatible with binder solutions or dispersions, such as gelatin, in water (at 25.degree. C.) containing as much as about 5 weight percent each of gelatin and latex "solids"; and PA1 (c) when dispersed in water have a discontinuous phase which consists essentially of polymeric particles which will absorb or otherwise receive hydrophobic compounds forced out of solution in the water-miscible solvent. PA1 (a) can be dissolved in (i.e., are "miscible" with) distilled water at 20.degree. C. to the extent of at least about 20 parts by volume of solvent in 80 parts by volume of water; PA1 (b) have boiling points (at atmospheric pressure) above about 20.degree. C.; PA1 (c) do not detrimentally react chemically with the loadable polymer latexes which are useful in the practice of this invention; PA1 (d) do not dissolve more than about 5 weight percent of such loadable polymer latices at 20.degree. C.; and PA1 (e) act as solvents for the organic fluors described hereinafter at least to the extent of 0.02 weight percent at 20.degree. C. for the secondary fluor and 1.0 weight percent for the primary fluor. PA1 (a) from about 25 to about 100 weight percent of a styrene monomer having the formula ##STR1## wherein R.sup.1 is hydrogen or methyl; R.sup.3, R.sup.4 and R.sup.6 are hydrogen or lower alkyl of 1 to 4 carbon atoms; R.sup.5 is hydrogen or with R.sup.4 constitutes the atoms necessary to complete a fused benzene ring; PA1 (b) from about 0 to about 95 weight percent of units derived from one or more ethenic monomers of the formula ##STR2## wherein R is hydrogen or alkyl containing 1 to 5 carbon atoms; R.sup.1 is hydrogen or methyl; and J is hydrogen, halogen, methyl, cyano when R.sup.1 is hydrogen, the group ##STR3## or the ester ##STR4## wherein R.sup.2 is an aliphatic group containing from 1 to 6 carbon atoms; and PA1 (c) from about 0 to about 10 weight percent of a hydrophilic ethenic monomer containing a sulfonic acid group, or an ammonium or alkali metal salt thereof; said ethenic monomer preferably having a molecular weight of at most about 300. PA1 (a) loadable polymeric latex; and PA1 (b) solution of fluor material(s) (in water-miscible solvent) which are intermixed in the required manner, are not believed critical. Thus, so long as some loadable latex particles are present in the solution during that interval of time in which the fluor is forced out of solution (because of the increasing hydrophilicity of the solution, as described above), some loaded latex particles will be created. For example, one embodiment of the present generic process involves, stepwise, PA1 (a) the introduction of a quantity of loadable latex which is not sufficient to affect the hydrophilicity of the solution of fluor to the extent necessary to force the fluor out of solution; and PA1 (b) adding enough water to the resulting mixture to affect the desired transfer of fluor from the water-miscible solvent into the latex particles. PA1 (a) the identity of the polymeric latex, fluor and water-miscible solvent; PA1 (b) the relative concentrations of fluor and polymeric dispersed phase in the respective materials to be mixed, as well as; PA1 (c) the relative amounts of latex and fluor solution.
U.S. Pat. No. 3,024,221 issued Mar. 6, 1962 discloses certain new sulfo esters of .alpha.-methylene carboxylic acids. No mention is made of the compounds being useful in scintillation counting compositions.